DEBT - Debt Obligations (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Debt Instrument [Line Items]
Less debt issuance costs	$ (43)	$ (46)
Total debt	4,948	4,933
Less current portion	500	442
Long-term debt, net of current portion	$ 4,448	$ 4,491
Weighted-average fixed-interest rate	4.00%	3.90%
Weighted-average variable-interest rate	6.00%	6.30%
Fixed-rate notes payable due through 2037
Debt Instrument [Line Items]
Total debt, gross	$ 127	$ 56
Fixed-rate PSP notes payable due through 2031
Debt Instrument [Line Items]
Total debt, gross	629	688
Fixed-rate EETCs payable due through 2027
Debt Instrument [Line Items]
Total debt, gross	801	864
Fixed-rate Japanese Yen denominated notes payable due through 2031
Debt Instrument [Line Items]
Total debt, gross	61	88
Variable-rate PSP notes payable due through 2030
Debt Instrument [Line Items]
Total debt, gross	61	0
Variable-rate notes payable due through 2037
Debt Instrument [Line Items]
Total debt, gross	1,316	1,283
Loyalty financing, variable-rate term loan facility due through 2031
Debt Instrument [Line Items]
Total debt, gross	746	750
Loyalty financing, fixed-rate notes due through 2031
Debt Instrument [Line Items]
Total debt, gross	$ 1,250	$ 1,250